In 1984, the United States, under the auspices of the Federal Communications Commission, adopted a standard for the transmission and reception of stereo audio for television. This standard is codified in the FCC's Bulletin OET-60, and is often called the BTSC (after the committee which proposed it) system, or the MTS (Multi-channel Television Sound) system.
Prior to the BTSC system, broadcast television audio was monophonic, consisting of a single “channel” or signal of audio. Stereo audio requires the transmission of two independent audio channels, and receivers capable of detecting and accurately recovering both of these channels. In order to meet the FCC's requirement that the transmission standard be ‘compatible’ with existing monophonic television sets (meaning, in other words, that mono receivers be capable of reproducing an appropriate monophonic audio signal from the new type of stereo broadcast), the BTSC committee adopted an approach which had worked for the FM radio world: the stereo Left and Right audio signals are combined to form two new signals, the Sum signal and the Difference signal, which are then modulated for broadcast.
Monophonic television receivers detect and demodulate only the Sum signal, consisting of the addition of the Left and Right stereo signals. Stereo-capable receivers detect and demodulate both the Sum and the Difference signals, recombining them to extract the original stereo Left and Right signals.
For transmission, the Sum signal directly modulates the aural FM carrier just as would a monophonic audio signal. The Difference channel, however, is first modulated onto an AM subcarrier located 31.768 kHz above the aural carrier's center frequency. The nature of FM modulation is such that background noise increases by 3 dB per octave, and as a result, because the new subcarrier is located further from the aural carrier's center frequency than the Sum or mono signal, additional noise is introduced into the Difference channel, and hence into the recovered stereo signal. In many circumstances, in fact, this rising noise characteristic renders the stereo signal too noisy to meet the requirements imposed by the FCC, and so the BTSC system mandates a noise reduction system in the Difference channel signal path.
This system, sometimes referred to as dbx-TV noise reduction (named after the company that developed the system) is of the companding type, comprising an encoder and decoder. The encoder adaptively filters the Difference signal prior to transmission so that its amplitude and frequency content will, upon decoding, appropriately hide (“mask”) noise picked up during the transmission process. The decoder completes the process by restoring the Difference signal to its original form and in so doing ensures that noise is audibly masked by the signal content.
The BTSC system also defines a Secondary Audio Programming (SAP) signal, an additional monophonic information channel often used to carry programming in an alternative language, reading services for the blind, or other services. The SAP channel is also susceptible to added noise during broadcast, and so the dbx-TV noise reduction system is used to encode and decode the SAP channel, as well as the aforementioned stereo signals.
The BTSC system is designed to provide audio signals of reasonably high fidelity, and as such its performance can be quantified with those parameters traditionally used to measure the quality of audio delivery systems. In particular, stereo separation is of prime importance, given that the delivery of stereo audio is the main reason for the development of the BTSC system.
As noted earlier, in order to maintain backwards compatibility with existing monophonic TV signals the BTSC system actually broadcasts a Sum signal and a Difference signal, each derived from the original Left and Right audio signals. At the receiver, the Sum and Difference signals are recombined to recover and reproduce the Left and Right signals. Accurate recovery by the receiver, and particularly recovery with good stereo separation, occurs if the various filters in both the broadcast and receiving equipment—especially those that comprise the dbx-TV encoder and decoder—comply closely with the ideal transfer functions defined in OET-60, the BTSC standards document. Inaccuracies in these filters result not just in poor stereo separation, but in degradation of other important audio parameters including frequency response, distortion, and dynamic amplitude accuracy.
The quality of the recovered SAP signal, too, is dependent on the accuracy of the various filters defined by the BTSC standard, again especially those within the dbx-TV encoder and decoder. Since the SAP signal is monophonic, stereo separation is not an issue, of course. However, other audio characteristics will degrade as a result of filter inaccuracies, just as they will with respect to the stereo Left and Right signals.
The BTSC standard defines all of the critical filters in terms of their analog filter transfer functions. While it is generally possible to design a digital filter so that either the magnitude or phase response of the digital filter matches that of an analog filter, as is well known matching both the amplitude and phase responses simultaneously can require prohibitively complex (and expensive) filter topologies.
With no compensating filters, the audio performance of the recovered signal may suffer dramatically. Thus, heretofore, the alternative has been to use simpler digital filters and either accept the reduced audio performance or add additional compensatory networks, usually in the form of all-pass filters, to attempt to correct at least some of the inevitable phase inaccuracies. In the latter case, the compensatory networks add complexity and cost. Further, because certain critical filters used in the dbx-TV encoder and decoder are dynamic in nature with transfer functions that change from moment to moment depending on signal content, fixed compensation networks are necessarily a compromise, and do not adequately track the filter errors as the various signals change over time.